It is well known in the prior art to provide a field shunt operation for a transit vehicle electric motor. The motor commonly operates from zero speed up to an intermediate base speed with a full field operation to provide a desired torque in relation to a load, and when the back EMF of the motor in full field operation builds up as a function of operating speed it becomes necessary to shunt the main field winding with a shunt field for a weakened field operation to enable more armature current for holding the desired torque to operate at greater speed above that intermediate base speed. When the field shunt operation is provided, since motor torque is proportional to the field flux times the armature current, in order to provide the same desired output torque an increase in the armature current is required since the field flux is now decreased.
It is known in the prior art to control the operation of one or more transit vehicle motors with a chopper apparatus as described in U.S. Pat. Nos. 3,559,009 of J. M. Mills, 3,543,121 of L. G. Miller and 3,530,503 of H. C. Appelo et al., and in a publication entitled "Alternative Systems for Rapid Transit Propulsion and Electrical Braking" in the March 1973 Westinghouse Engineer at pp. 34-41. It is known in the prior art to provide a full field and a shunt field operation of a traction motor for the control of dynamic braking as disclosed in U.S. Pat. No. 3,569,811.
It is known in the prior art as shown by U.S. Pat. No. 4,095,153 of T. C. Matty et. al to utilize a microprocessor chopper control of regenerative brake current buildup by providing an upper current limit to control chopper ON operation and a lower current limit to control chopper OFF operation.